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Without even realizing it, "The Food Lab," has really started to become "The Meat Lab," which is unfortunate, because I love vegetables (particularly spring vegetables—hello ramps!) even more than I love meat.

This week, we're going to remedy that by taking a look at the lowly salad, if only for the sake of my poor wife's waistline (I still love you dear).

For me, the big question about vinaigrettes has never really been "how?" but "why?"

Is emulsifying the oil and acid really necessary? Does adding the olive oil and the vinegar to the salad bowl individually really make for an inferior salad? Could every red-sauce Italian joint that serves salad with a side of oil and vinegar be wrong?

Well, stranger things have been true. I decided that a bit of hard-core kitchen work was in order.

Vinaigrette: Defined

a cold sauce or dressing made from a mixture of vinegar, oil, pepper, and salt, to which various flavorings may be added.

That's a starting point, but I think for most practical definitions, a vinaigrette must also be wholly or at least partially emulsified.

What exactly is an emulsion? At its most basic, it's what you get when you force two things that don't easily mix to form a homogeneous mixture. In cooking, this most often occurs with oil and water (and for all intents and purposes, vinegar or lemon juice can be considered water, as it behaves in the same way). Put them in a container together, stir them up, and eventually, like cats and dogs, they will separate and stick with their own kind.

There are a couple of ways around this.

The first is to disperse one of the two—the oil, say—into fine enough droplets that water can completely surround it. Kind of like putting a single cat inside a ring of dogs—there's no way for it to escape and rejoin its fellow feline friends. A common example of this kind of emulsion is homogenized milk, in which whole milk is forced at high pressure through a fine screen, breaking up its fat molecules into individual droplets that are suspended in the watery whey.

An easier way to form an emulsion is to add an emulsifying agent known as a surfactant.

Remember that cartoon CatDog? The one with the head of a cat on one side and the head of a dog on the other? Well, CatDog was kind of like a surfactant: he's got something that's attractive to both cats and dogs, which makes him a kind of feline-canine ambassador, allowing them to mix together a little more easily.

Culinary surfactants are molecules that contain one end that is attractive to water (hydrophilic), and one that is attractive to oil (hydrophobic). Common kitchen surfactants include egg yolks, mustard, and honey.

It's easy to see the work of a surfactant in action.

The container on the left contains oil and balsamic vinegar mixed in a ratio of 3:1. The container on the right has the same ingredients, with the addition of a small amount of dijon mustard. Both containers were sealed and shaken vigorously until they looked homogeneous. I then allowed them to rest at room temperature for 5 minutes. As you can see, the container without the mustard separated much more rapidly than the container with mustard.

At this point, you're probably thinking what I'm thinking: this is all very neat, but what difference does it make to my salad?

Good question.

Oil Slicks

My next order of business was to examine what happens when vinegar and oil are added to greens. I'd always been under the impression (and I'm not the only one) that a dressed salad eventually wilts because the acid in the vinegar attacks the leaves.

To test this theory, I dressed half an ounce of fresh salad greens with 1 teaspoon of distilled white vinegar (5% acetic acid), another half ounce with plain water (as a control), and a third half ounce of leaves with olive oil, then let them sit at room temperature for ten minutes.

Surprise! Turns out that vinegar is not the culprit at all.

The greens dressed with plain oil wilted significantly faster than those dressed in vinegar. In fact, the vinegar-coated greens fared pretty much just as well as those dressed in water!

The truth is that salad greens, like any leaf, spend their time exposed to the elements, and as such, need to protect themselves from the rain. They do this via a thin, waxy cuticle. It's like a little built-in raincoat for the leaf. On the other hand, this oily cuticle makes it very easy for the olive oil to penetrate the spaces between cells (dogs and dogs stick together, remember), causing damage to the leaf.

As a further test, I then dressed another batch of salad greens in a vinaigrette that I constructed without any surfactant (i.e. oil and vinegar, mixed as well as I could manage). I took an up-close-and-personal look at the results, and what I saw was this (above, right): drops of vinegar suspended above the surface of the leaf by larger drops of oil, like little blobs sitting in bean bag chairs. Lifting the leaf up between my fingers prompted a cascade of vinegar to tumble back into the bowl, while the oil continued to cling tenaciously to its surface.

A ha, I thought. This must be the key.

I set up one last experiment, this time dressing two 1-ounce portions of salad side-by-side. The first was dressed with a homogenized mixture of 1 tablespoon olive oil, 1 teaspoon vinegar, and 1/2 teaspoon dijon mustard. The second was dressed with the same mixture, minus the mustard. After tossing the greens, I immediately placed them inside funnels, which I set over a couple of small glasses in order to catch any drippings.

Almost immediately, the non-mustardy batch on the right started dripping a steady trickle of vinegar into the cup, while the well-emulsified dressing on the left stayed firmly in place.

After only ten minutes, the right cup had nearly a full teaspoon of vinegar collected in its bottom—almost the entire amount that I had put on the salad in the first place—and was starting to drip a few drops of oil as well. The cup on the left had shed at most a dozen drops.

Moral? Unless you emulsify your vinaigrette, you end up with a pile of leaves dressed in oil, and a pool of vinegar at the bottom of the salad bowl, completely destroying the flavor of the sauce. An emulsified vinaigrette however, uses the power of surfactants to help both oil and vinegar cling tightly to the leaves. Balanced flavor in every mouthful.

Looking at the greens themselves was even more revealing.

The salad dressed in the badly emulsified vinaigrette showed definite signs of wilting, while the salad dressed in the proper vinaigrette was still crisp and fresh-tasting. Apparently, straight-up oil is much more damaging to leaves than an oil-vinegar mixture.

Obsessive-Emulsive

The last few questions that still needed answering: ratio, the best surfactant for the job, and mixing method.

For ratio, the classic proved to be best: two to three parts oil to one part vinegar. It forms the strongest emulsion. If you'd like your vinaigrette less acidic, you can replace part of the vinegar with straight water and get equally stable results.

Mustard is the most common surfactant, and it works best when you have at least 1 teaspoon per tablespoon of vinegar (you can add more if you'd like). Mayonnaise works even better, easily forming a creamy sauce, though it lacks the pleasant tang of mustard. For a sweeter dressing (say, on a beet salad or an asparagus salad), honey also works very well.Try adding honey and toasted crushed nuts to a basic vinaigrette. It rocks in more ways than one.

As for mixing? Some advocate slowly whisking in the oil. Others shake it up in a jam jar. Still others insist on the blender.

Well, after testing, I found that not surprisingly, a blender will give you the tightest emulsion (I made a blended vinaigrettes that stayed emulsified for a whole week), while the shake-it-in-a-jar version will be the weakest, lasting around 30 minutes or so.

But the truth of the matter is: your vinaigrette only needs to stay stable for the length of time it takes you to eat a salad. So if it takes you a week to eat a salad,* then by all means, whip out the blender.

*The only situation I can imagine this happening is if my mom had served us more salads when we were kids. She had a no-leaving-the-table-until-you've-finished-and-I-don't-care-how-long-it-takes-or-even-if-you-vomit policy.

Personally, I put the ingredients for my vinaigrette into a 1-pint squeeze bottle in the fridge and shake it up right before I use it. Or, as is more often the case, take it out of the fridge, realize that once again my wife has finished off all but the last drop and replaced the bottle, hoping I'd notice and make more. Of course, I always do, but one of these days, I'm going to leave out the mustard, just out of spite.

We'll see how much you like your soggy salad then, dear...

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J. Kenji López-Alt is the Managing Culinary Director of Serious Eats, and author of the James Beard Award-nominated column The Food Lab, where he unravels the science of home cooking. A restaurant-trained chef and former Editor at Cook's Illustrated magazine, his first book, The Food Lab: Better Home Cooking Through Science is a New York Times Best-Seller, the recipient of a James Beard Award, and was named Cookbook of the Year in 2015 by the International Association of Culinary Professionals.

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